Aqueous polymer dispersions are widely used as binders for coatings or paints. It is often desirable that such coatings or paints have a high hiding capacity to enable the paint to completely conceal the undersurface while utilizing a minimal application of the paint. Titanium dioxide is often the pigment of choice for imparting hiding properties to coating compositions like paints. In paints titanium dioxide is an expensive component in the formulation of the coating composition. Therefore it is an objective to achieve the desired hiding effect in a coating composition while using as little titanium dioxide as possible.
The hiding power of a coating is among others based on light scattering by the surfaces of the pigment particles. The average particle size and particle size distribution of the pigments used for hiding purposes are already highly optimized for maximum scattering by the manufacturers of pigments like titanium dioxide. But light scattering is also a function of the spacing of the titanium dioxide particles in the dried coating. Maximum light scattering is obtained, only if the particles are spaced apart from each other so that there is a minimal interference between the light scattering of neighboring particles. Light scattering can also be optimized via inclusion of certain sizes of air voids within the coating. Often air voids are unintentionally or intentionally part of a coating film due to the relative concentration of pigment and binder or more specifically, the pigment volume concentration of the coating formulation.
In the state of the art a number of techniques have been proposed to improve the hiding powder of coatings with as little titanium dioxide as possible. U.S. Pat. No. 4,771,086 discloses a process wherein nonionic monomers are polymerized in the presence of a nonionic emulsifier and in the presence of TiO2 particles. The naturally agglomerated TiO2 particles are dispersed in situ during polymerization. U.S. Pat. No. 5,385,960 discloses a process comprising the admixture of a dispersion of TiO2 particles with a polymer dispersion of a polymer having dihydrogen phosphate functional groups for a better adsorption of the TiO2 particles. U.S. Pat. No. 6,080,802 describes a further process for obtaining an aqueous polymer dispersion with TiO2 particles which are adsorbed on the surface of the polymer particles.
Adsorption is achieved with polymer latex particles which have the same sign of the surface charge like the TiO2 particles. In the process of U.S. Pat. No. 7,579,081 a first polymer is attached to the pigment particles via a silane residue. In the presence of the thus obtained particles a monomer emulsion is polymerized forming an envelope around the particles obtained in the first step.
A similar process is disclosed in US 2010/0056668 A1, which describes a process comprising dispersing pigment particles in an aqueous medium with aminophosphorus acid-functional first polymer, and performing an emulsion polymerization in the presence of the dispersed pigment particles. In the thus obtained product the dispersed pigment particles are encapsulated by the second polymer.
In the process disclosed in WO 2012/116025 A1 a polymerization is performed in an aqueous dispersion comprising TiO2 particles, an amphoteric polymer and sodium styrene sulfonate. The product is an aqueous dispersion of TiO2 particles which are encapsulated with the polymer.
In EP 2 426 166 A2 an aqueous composition is described comprising TiO2 particles which are adsorbed on emulsion polymer particles with phosphoric acid monomer units.
All of these techniques of the state of the art have the same inherent problem of polymer dispersions containing high proportions of pigment, namely colloidal stability. Although it is known to disperse and chemically or physically couple a pigment in a polymer dispersion to enhance the dispersion of the pigment, this approach is severely limited by the need to maintain the colloidal stability of the resulting product. Pigments are typically of much higher density than polymers and thus cannot easily be colloidally stabilized resulting in handling issues related to a slurry. In addition the improvement of the hiding power of pigment particles by an holospheric approach which means enveloping the pigment particles with a polymer shell is limited in the capacity of creating voids between the polymer particles. In addition mostly hard polymers with a high Tg are used which have very low binding capability.